(a) Field of the Invention
This invention relates to the reduction of losses by evaporation particularly, for example, in regard to evaporative losses in the case of petrol stored in bulk tanks and bulk storage depots but also in relation to the prevention of evaporative losses from any liquids being stored. The invention naturally has particular application in regard to costly liquids which are subject to evaporative losses and a further example in this regard is the storage of pharmaceutical alcohol. Any volatile liquid will be of interest in the context of this invention and further examples are methylene chloride, methyl and ethyl alcohol, acetone and many others.
(b) Description of the Prior Art
In the bulk storage of volatile liquids at ambient temperature and pressure evaporative losses are a natural and unavoidable consequence. It is not economically justifiable to construct bulk storage tanks so as to sustain a sufficient pressure on the stored liquid to prevent evaporative losses nor is it economically justifiable to cool the stored liquid so as to reduce or eliminate evaporative losses by this means. In practice the free space above the stored liquid and under the roof of the tank under continental climatic conditions can reach a temperature considerably higher than the temperature of the body of the liquid itself. Such bulk storage tanks are vented at a small pressure above ambient by means of a safety valve for example set below 14 kPa. Even at this small pressure a substantial evaporative loss can occur especially in continental climatic conditions in which the temperature of the vapour in the tank above the free surface of liquid can rise for example to 60.degree. C. (For example at that temperature, petroleum would have a vapour pressure approaching 90 kPa). As a result vapourised fuel is continuously vented in the case of a bulk fuel storage tank.